1. Field of the Invention
This invention relates to a light guide which transmits light from one end face thereof to the other end face thereof.
2. Description of the Prior Art
There has been put into practice in various optical devices a light guide for transmitting light from one end face positioned close to a light emitting position to the other end face connected to a photodetector or the like. For example, such a light guide is used in a radiation image recording and reproducing system disclosed, for instance, in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395.
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In said radiation image recording and reproducing system, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal by a photodetector, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like. The light guide is used in a radiation image read-out apparatus of the radiation image recording and reproducing system as a means for transmitting the light emitted by the stimulable phosphor sheet to the photodetector such as a photomultiplier. An example of the radiation image read-out apparatus is shown in FIG. 16.
In FIG. 16, a stimulating light beam 101a of a predetermined intensity is emitted from a stimulating light source 101 to impinge upon a light deflector such as a galvanometer mirror 102. The stimulating light beam 101a is deflected by the galvanometer mirror 102 to impinge upon a stimulable phosphor sheet 103 positioned below the galvanometer mirror 102 so that the stimulable phosphor sheet 103 is scanned by the stimulating light beam 101a in the main scanning direction, i.e., in the width direction of the sheet 103 as indicated by arrow A. While the stimulating light beam 101a impinges upon the stimulable phosphor sheet 103, the sheet 103 is conveyed in the sub-scanning direction as indicated by arrow B, for example, by an endless belt device 109. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 103 is two-dimensionally scanned by the stimulating light beam 101a.
As the stimulable phosphor sheet 103 is scanned by the stimulating light beam 101a the portion of the sheet 103 exposed to the stimulating light beam 101a emits light having an intensity proportional to the stored radiation energy. The light emitted by the stimulable phosphor sheet 103 enters a transparent light guide 104 from its light input face 104a positioned close to the sheet 103 in parallel to the main scanning line. As shown in FIG. 17, the light guide 104 has a flat-shaped front end portion 104b positioned close to the stimulable phosphor sheet 103 and is shaped gradually into a cylindrical shape towards the rear end side to form a substantially cylindrical rear end portion 104c which is closely contacted with a photomultiplier 105. The light entering the light guide 104 from its light input face 104a is guided inside of the light guide 104 by total internal reflection up to the rear end portion 104c, and is received by the photomultiplier 105 by way of a filter (not shown) which selectively transmits the light emitted by the stimulable phosphor sheet 103. Thus the light emitted by the stimulable phosphor sheet 103 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 105. The electric image signal thus obtained is sent to an image processing circuit 106 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or is stored in a magnetic tape 108.
The light input face of the light guide must extend along the main scanning line, the light output end must be in such a shape as to permit connection to the photomultiplier having a small width, and the light guide must be uniform in width and thickness since light is transmitted through the light guide by total internal reflection. Accordingly, the light guide is formed by rolling an end portion of a rectangular light guide sheet material to conform to the shape of the photomultiplier as shown in FIG. 17. The conventional light guide having such a shape is disadvantageous in that the dimension in the direction perpendicular to the light input face is large to enlarge the size of the overall radiation image read-out apparatus.
When the light guide for the radiation image read-out apparatus is formed of a bundle fiber comprising a plurality of optical fibers bundled into a flat plate shape, the overall size of the apparatus can be reduced since the bundle fiber can be relatively freely deformed. However since the light input face of the bundle fiber is formed by a plurality of light input faces of the optical fibers forming the bundle fiber, the spaces which cannot contribute to transmission of light are formed between the light input faces of the respective optical fibers. Therefore, the light transmitting efficiency of the bundle fiber is as low as several percents and is 50% at most, and the bundle fiber cannot be practically used for transmitting weak light. Further since the bundle fiber is formed of a plurality of optical fibers as described above, the light transmitting efficiency substantially fluctuates in the light input face, and shading is generated at small intervals in each scanning line due to the fluctuation of the light transmitting efficiency. Since this shading is increased as the distance between the light input face of the bundle fiber and the scanning position is reduced, influence of the shading can be lowered by positioning the light input face of the bundle fiber at a large distance from the scanning position. However, when the distance between the scanning position and the light input face of the bundle fiber is enlarged, the amount of light entering the bundle fiber is inherently reduced. In the case where the reflected light is received by the bundle fiber, reduction in the amount of light can be compensated for by increasing the amount of the scanning light beam. However, in the case of the aforesaid radiation image read-out apparatus, increase in the intensity of light emitted from the stimulable phosphor sheet with increase in the amount of stimulating light beam is just slight. Accordingly, in the aforesaid radiation image read-out apparatus, the bundle fiber cannot be practically used as the light guide.